You made a claim here now support it. We're still waiting for you to show us a source.
Here you go. The follwing BC instructions clearly allow parents to classify their race however they want. They can check one of the boxes, but they can also check "other" and write in their own classification:"
http://www.cdc.gov/nchs/data/dvs/birth11-03final-ACC.pdf
Given that Pan-African nationalists generally detest the terms "negro" or "black," I don't see why anyone would find is at all surprising that Obama Sr. would check "other" and classify himself as "African."